1. Field of the Invention
This invention relates to an increased-security security document system, and more particularly relates to an identification card having a polarization-altering overlay over at least one field; such field when viewed through a properly polarized viewer provides both tamper evidence and a view of otherwise non-visible security data.
2. Description of Related Art
Several methods to provide security features to prevent counterfeiting of a variety of valuable documents are known, including monetary printed documents (such as paper currency, stock certificates and bonds), identification documents (such as drivers' licenses and passports) and multi-purpose documents (such as credit cards, debit cards and key cards). The importance of security in such documents is emphasized not only by the financial aspects of forged financial instruments, but also by the fact that traffic accidents caused by under-age drivers and frequent traffic violators with suspended licenses who procure alcohol using false drivers' licenses and other identification documents are major contributors to fatalities, injuries and property damage.
Methods of making security documents tamper-resistant go back to prehistory, including baked ceramic tablets with cuneiform writing and seals, which made altering of information difficult without also breaking the seal. In recent years, several more advanced techniques for fabricating security documents have been practised. Whereas certain features of these prior-art methods are noteworthy, by and large they have not been successful in reducing counterfeiting due to the ease with which it is possible to either simply duplicate original documents or decipher the precise elements of a particular method that give it its uniqueness. In many cases, the prior-art methods have not been sufficiently economical to implement. Below I briefly describe the known methods, and then summarize their limitations.